On my Hitachi 203f2 edm i had the problem of keeping the waterpressure constant. Sometimes the upper surface of the workpiece is not 100% parrallel with the bottom part and with a fixed upper nozzle it spells PROBLEMS. I made my self a floating upper nozzle. The water pressure is now constant, better flushing and i can cut FASTER. According to the manual i'm suppose to get a max feed rate of 12.3 to 13.1 mm/min (0.48 - 0.51 inch/min) on 10mm thick alluminium i equaled it on medium settings!!!
I know that the guys with Charmilles, Agies and the new Fanucs don't have this problem.
A more expensive solution would be to buy a submersible edm.
Klox